This invention relates to impellers and to pumps for pumping molten metal which employ the impellers.
Pumps used for pumping molten metal typically include a motor carried by a motor mount, a shaft connected to the motor at one end, and an impeller connected to the other end of the shaft. Such pumps may also include a base with an impeller chamber, the impeller being rotatable in the impeller chamber. Support members extend between the motor mount and the base and may include a shaft sleeve surrounding the shaft, support posts, and a tubular riser. An optional volute member may be employed in the impeller chamber. Pumps are designed with shaft bearings, impeller bearings and with bearings in the base that surround these bearings to avoid damage of the shaft and impeller due to contact with the shaft sleeve or base. The shaft, impeller, and support members for such pumps are immersed in molten metals such as aluminum, magnesium, copper, iron and alloys thereof. The pump components that contact the molten metal are composed of a refractory material, for example, graphite or silicon carbide.
Pumps commonly used to pump molten metal may be a transfer pump having a top discharge or a circulation pump having a bottom discharge, as disclosed in the publication xe2x80x9cH.T.S. Pump Equation for the Eightiesxe2x80x9d by High Temperature Systems, Inc., which is incorporated herein by reference in its entirety.
One problem that such pumps encounter is that they may be damaged by solid impurities contained in the molten metal including chunks of refractory brick and metal oxides (e.g. aluminum oxides). If a piece of hard refractory material becomes jammed in the impeller chamber it may destroy the impeller or shaft, and result in the expense of replacing these components. Chunks of refractory material such as brick with a higher specific gravity than the metal are disposed at the bottom of the vessel. Aluminum oxides with a lower specific gravity than the molten metal rise to the surface of the bath. Refractory material that has a specific gravity approximating that of the molten metal may be suspended in the bath. Refractory impurities in the molten metal are also a problem since, if not removed, they result in poor castings of the metal and potentially defective parts. Removing impurities from the molten metal bath is a hazardous process. A long steel paddle with an end that is in the shape of a perforated spoon is used to remove the impurities. To remove impurities with the paddle, workers need to come close to the molten metal at an area where temperatures may exceed 120 degrees Celsius. Although workers wear protective gear, they may be injured by splatters of metal. At the least, workers face a difficult task in removing the impurities, which they carry out in a two-step process, spooning the material upward from the bottom of the vessel and skimming the material from the surface. Each step typically lasts about 10-15 minutes. Removing the material from the bottom is carried out at least once a day and skimming is carried out at least once every eight hours. Removing impurities from the molten metal is a hazardous, costly, but necessary, process using traditional pump and impeller designs.
A second main design concern with a molten metal pump is clogging. Any impeller with an internal path for molten metal travel is susceptible to clogging, caused by solid pieces becoming lodged in the impeller and between the impeller and base. As mentioned, clogging can cause damage to the impeller and generate expensive down-time and repairs. Some impeller designs attempt to solve this problem with specifically designed passages. A passage with an entrance less in diameter than the exit may help to reduce clogging, as alleged in U.S. Pat. No. 5,785,494 to Vild. Particles which are small enough to enter the entrance to the passage in theory pass easily through the exit of the passage.
A third main design concern with a molten metal pump is efficiency. The geometric design of a pump impeller primarily defines the fluid dynamic characteristics of the pump. The impellers of the U.S. Pat. No. 5,785,494 which have internal passages wherein the entrance diameter of each passage is less in diameter than the exit diameter, have a design which results in losses in pump efficiency and higher operating costs. Internal passages of such impellers are configured to permit travel along a direction of the pump axis and then in a radial direction. Despite reducing clogging, impellers of this design may suffer significant efficiency losses.
There is a need for an impeller and pump for pumping molten metal not prone to clogging which offer high efficiency operation, low maintenance cost, and safe operating conditions for personnel.
The present invention is directed to a pump for pumping molten metal with an impeller. One aspect of the invention utilizes an impeller comprising internal molten metal passages which are configured to increase the efficiency of the impeller. The travel of molten metal through the passages is at an angle to the central rotational axis of the impeller. The geometry of the passages further prevents clogging. The impeller may include optional stirrer passages which are configured and arranged to enable the impeller to cause solid matter in the molten metal to move toward an upper surface of the bath.
As defined herein, the term passage means a tunnel in which the flow of molten metal may be controlled so as to travel along a defined, relatively narrow path. Vanes are defined as discrete surfaces of an impeller, extending from near a lower portion of the impeller along its rotational axis to near an upper portion of the impeller, which do work to move molten metal when the impeller is rotated. Cavities are defined herein as the regions between adjacent vanes and have a height which is much greater than the largest cross-sectional area of the impeller passages.
In general, the present invention is directed to pumps for pumping molten metal including a motor and a shaft having one end connected to the motor. An impeller is connected to the other end of the shaft which extends along a longitudinal axis, the impeller being constructed in accordance with the present invention. A base has a chamber in which the impeller is rotatable.
One embodiment of the present invention is directed to an impeller made of a non-metallic, heat resistant material comprising a body having a generally cylindrical shape. The impeller includes a central rotational axis, and first and second generally planar end faces extending transverse to the central axis. A side wall extends between the first and second faces. A plurality of passages have inlets circumferentially spaced apart from each other on the first face, and outlets at the side wall. Connecting portions of the passages extend between the inlets and the outlets transverse to the central axis.
More specifically, each passage extends at an angle to the central axis along substantially its entire length and perimeter. More preferably, the side surface of each passage intersects the impeller sidewall at a downward angle relative to an axis extending radially from the central axis. The angles of each passage to the central axis are intended to provide the impeller with a high operating efficiency. The passages are preferably reverse pitched relative to a direction of rotation of the impeller.
The impeller may include stirrer passages in one of the faces circumferentially spaced apart from each other. The stirrer passages are configured and arranged to enable the impeller to cause solid matter in the molten metal to move toward an upper surface of the bath. Each stirrer passage extends at an angle to the central axis along substantially its entire length and perimeter. The stirrer passages in the cylindrical bodied impeller may be enlarged to have a cross-sectional area approximating that of the other passages. The stirrer passages thus function as infeed passages for the molten metal and the pump may be referred to as a top-and-bottom feed pump.
The sizes of the passages in the cylindrical body impeller may be varied. In a bottom feed pump large passages (similar to the size of the passages now shown in the top face in FIG. 2) may have inlets in the bottom face of the impeller. In such pump the upper face may have no passages, relatively small cross-sectional area stirrer passages or infeed passages having a size approximating that of the lower passages. Thus, the pump may be modified, by changing the size and location of the passages in the cylindrical body impeller, so as to be one of the following: top feed; bottom feed; top feed or bottom feed with stirrer passage inlets in the opposite end face; and top-and-bottom feed.
Another embodiment of the present invention is directed to a vaned impeller made of a non-metallic, heat resistant material. The impeller includes a generally cylindrical hub portion extending along a central rotational axis, and first and second bases spaced apart from one another along the central axis at opposing end portions of the impeller and extending transverse to the central axis. Vanes extend outwardly from the central hub portion between the first and second bases. Cavities of the impeller are each disposed between the first and second bases and between adjacent vanes. The impeller top end face (in the case of a top feed pump) includes a plurality of passages. The inlets of the passages are circumferentially spaced apart from each other in the first end face, and the passages terminate at the cavities of the impeller. The passages preferably extend from the top end face, through the first base portion and terminate at the cavities, all the while extending transverse to the central axis. The invention is also directed to a pump which employs this vaned impeller.
More specifically, each passage extends through the first impeller base at an angle to the central axis along substantially its entire length and perimeter. Further, each passage extends to the cavity at a downward angle relative to an axis extending radially from the central axis. The angle of each passage to the central axis is effective to provide the impeller with a high operating efficiency. The passages are preferably reverse pitched relative to a direction of rotation of the impeller.
A bearing member may be disposed around the impeller first end face and second end face. The first and second bases may be integrally formed with the body. Alternatively, the first and second bases may comprise a plate formed separately from the impeller and fastened to it. Each stirrer passage extends at an angle to the central axis along substantially its entire length and perimeter, and terminates in a cavity. The stirrer passages are configured and arranged to enable the impeller to cause solid matter in the molten metal to move toward an upper surface of the bath.
The vaned impeller of the invention is preferably formed so that the lower passages have a large size approximating that of the other (e.g., upper) passages. Thus, the passages in the top face and the passages in the bottom face act as infeed passages which enable molten metal to be drawn into the pump from below and above the base. This enables the pump which employs the vaned impeller to function as a top-and-bottom feed pump.
The sizes of the passages in the vaned impeller may be varied. In a bottom feed pump large passages (similar in size to the passages shown in the bottom face in FIG. 6) may have inlets in the bottom face of the impeller. In such pump the upper face may have no passages, relatively small cross-sectional area stirrer passages or infeed passages having a size approximating that of the lower passages. Thus, the pump may be modified, by changing the size and location of the passages in the vaned impeller, so as to be one of the following: top feed; bottom feed; top feed or bottom feed with stirrer passage inlets in the opposite end face; and top-and-bottom feed.
The present invention presents advantages compared to typical pumps and impellers for pumping molten metal. Pumps for pumping molten metal are prone to clogging, which occurs when solid particles enter and lodge in the impeller between the impeller and base. Pumps in the prior art have attempted to address clogging with the use of internal passages having inlet diameters smaller in size than exit diameters, as in the case of the U.S. Pat. No. 5,785,494. Solid particles which are small enough to enter the entrance to the passage in theory pass through the larger exit of the passage. Nevertheless, it is believed use of the impeller of the U.S. Pat. No. 5,785,494 results in losses in pump efficiency and higher operating costs.
In contrast, one aspect of the present invention uses internal passages that permit molten metal travel at an angle to the central rotational axis along substantially the entire length and perimeter of the passage. Rotation of these passages imparts forces to the molten metal which improve the efficiency of the pump. Further, stirrer passages of the present invention, if used, may provide forces that act upon molten metal such as below the pump base in a top feed pump. Rotation of the stirrer passages is believed to enable particles, especially those suspended particles having approximately the specific gravity of the molten metal, to rise toward the surface of the bath. Therefore, when pumping molten metal according to the present invention, an improvement of pump efficiency, without clogging, is realized.
In addition, the vaned impeller of the invention moves molten metal differently than in the U.S. Pat. No. 5,785,494 in that it employs much shorter passages which are only in the upper and lower bases and which preferably extend at an angle to the central axis along substantially their entire length and periphery. In the vaned impeller of the invention the passages terminate in the much larger cavities formed between vanes of the impeller. The impeller relies on vanes to perform most of the work on the molten metal as do conventional vaned impellers, but utilizes the infeed or stirrer passages for straining to avoid clogging. In contrast, the U.S. Pat. No. 5,785,494 states that a vaned impeller is disadvantageous in that molten metal flow is difficult to control between adjacent vanes of the impeller. The U.S. Pat. No. Des. 5,785,494 relies solely on passages or tunnels to perform work to move the molten metal and is disadvantageous in that the passages extend along the central axis and thus are believed to provide the impeller with lessened efficiency. Moreover, the impeller of the U.S. Pat. No. 5,785,494 employs a sidewall which is lacking in the inventive vaned impeller. The inventive vaned impeller enables a far greater volume of molten metal to be acted upon by its vanes than do the narrow passages of the U.S. Pat. No. 5,785,494.
Many additional features, advantages and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows. It should be understood that the above Summary of the Invention describes the invention in broad terms while the following Detailed Description describes the invention more narrowly and presents specific embodiments which should not be construed as necessary limitations of the broad invention as defined in the claims.